This invention relates to a method and device for controlling an induction motor which can be applied to the spindle driving or servomotor driving of tooling machines and, more particularly, which can control the position, speed and acceleration of the induction motor in a highly responsive and precise manner to thereby obtain a stable operating performance in response to temperature changes and the like.
There have been proposed various control methods for induction motors. Typically, such prior art methods include a voltage/frequency (V/F) control method and a vector control method.
FIG. 1 is a schematic view of a device for carrying out the prior art voltage/frequency (V/F) control method. This method maintains the excitation current of the induction motor at a constant level by making the ratio of the voltage and the frequency constant.
FIG. 2 is a block diagram of a device for realizing the prior art vector control method. In this method, a speed controlling unit 1 outputs a a difference between a secondary current command I.sub.2CMD based on a speed command S.sub.CMD and a speed S.sub.M of an induction motor (IM) 13, and a slip frequency generating unit 3 outputs a slip frequency S.sub.S based on the above secondary current command. A field speed S.sub.F is obtained by adding the slip frequency S.sub.S and the speed S.sub.M of the induction motor 13 detected by a speed detector 14. A field current command unit 2 outputs a field current command I.sub.FCMD based on the input speed S.sub.M of the induction motor 13. Then a current command unit 7 obtains a rotation angle .theta..sub.F of the field current and the rotation angle .theta..sub.2 of the secondary current based on the field speed S.sub.F, induces a primary current command I.sub.1CMD by composing in format the secondary current command I.sub.2CMD and the field current command I.sub.FCMD as shown in FIG. 3, converts the thus obtained primary current command into three-phase current commands, i.e. U-phase current command I.sub.UCMD, V-phase current command I.sub.VCMD, and W-phase current command I.sub.XCMD, and then supplies the three-phase current commands to a current controlling unit 8. The current controlling unit 8 controls an applied electric current based on a difference between these current commands and current detected feed back signals to thereby control the induction motor 13 via a power amplifier 9.
The vector control method for the induction motor 13 is summarized as described above. The vector control method also decreases the field to current corresponding to the speed S.sub.M of the induction motor 13 in order to extend the controllable speed range, as well as to control the motor with at a constant power, and changes the secondary current command I.sub.2CMD and the slip frequency S.sub.S to be counterproportional to the normal field current command I.sub.FCMD, but a detailed description of these controls is omitted herein.
The above mentioned prior art voltage/frequency (V/F) control method is problematic in that although it can control the motor with a variable speed, it generally has a slow response time in the case when slip occurs due to an excessive load and the speed changes according to the slip frequency or when the load changes. Theoretically, the prior art vector control method would control an induction motor if it assumes such a motor as an ideal model and controls it precisely. In practice, however, the controllability of an induction motor often raises problems partly because copper is used as the secondary conductor in induction motors, the secondary resistance of which changes by approximately 40% in response to a temperature change of 100.degree. C., and partly because of the imperfection of the control system therefor, whereby the secondary current cannot be controlled properly.
Problematic aspects of the prior art vector control method will be discussed below in further detail.
The operational principle of the induction motor 13 will first be discussed. The torque T.sub.M generated by the induction motor 13 can be expressed by the equation (1) if the field current is denoted as I.sub.F, the secondary current as I.sub.2, and the torque coefficient as K.sub.1. EQU T.sub.M =K.sub.1 .multidot.I.sub.F .multidot.I.sub.2 ( 1)
The induced voltage V.sub.2 is expressed by the equation (2) if the slip frequency is denoted as S.sub.S, and the secondary induced voltage coefficient as K.sub.2. EQU V.sub.2 =K.sub.2 .multidot.I.sub.F .multidot.S.sub.S ( 2)
The secondary induced voltage V.sub.2 causes a rotor conductor of the induction motor 13 to rotate at the speed of the slip frequency S.sub.S. If the secondary time constant T.sub.2 =L.sub.2 /R.sub.2 (L.sub.2 : secondary inductance, R.sub.2 : secondary resistance) is sufficiently smaller than the slip frequency, the secondary current I.sub.2 may be expressed as a DC motor as shown in the equation (3). EQU I.sub.2 .apprxeq.V.sub.2 /R.sub.2 .multidot.I.sub.F .multidot.S.sub.S /R.sub.2 ( 3)
The speed controllability of the induction motor 13 depends on the torque controllability of the induction motor 13. The response of the induction motor 13 relative to the field current I.sub.F is generally greater than that of the secondary current I.sub.2 by at least a factor of ten. Therefore, the controllability of the induction motor 13 is proportional to the controllabilty of the secondary current according to the equation (1). Then, one must consider the controllability of the secondary current. As the response of the field current I.sub.F cannot be satisfactorily large as indicated by the equation (3), it is understood that the torque of the induction motor 13 can be controlled by controlling the slip frequency S.sub.S. The sliding frequency command S.sub.SCMD is inferentially computed by the slip frequency generating units 3 based on the equation (3) as described in relation to FIG. 2, and expressed as the equation (4) below. ##EQU1##
The equation (4) indicates that when the secondary resistance R.sub.2 varies by approximately 40% due to the temperature change of 100.degree. C., and the temperature on the rotor changes due to a certin driving condition of the induction motor 13, it becomes impossible to issue a precise torque command. This raises the first problem.
When the inferential computation of the slip frequency command S.sub.SCMD includes some errors, what compensation is provided by the vector control method? Referring to FIG. 2, if it is assumed that the secondary current becomes insufficient due to an error in the slip frequency command S.sub.SCMD, the three-phase current control loop, in accordance with the negative feedback of the three-phase primary current, tends to increase the voltage between the terminals of the induction motor 13 and the primary current. However, the correlation between the voltage between the terminals of the induction motor 13 and the secondary current is small from the beginning, and therefore the field current I.sub.F expressed by the equation (1) increases gradually. The increase in the field current causes a secondary reactive current which is not perpendicular to the magnetic field. The secondary active current perpendicular to the magnetic field can be compensated only by a slow increase in the field current I.sub.F. The second problem of the method therefore lies in that the three-phase primary current control loop cannot sufficiently control so as to compensate the error caused in the inferential computation of the slip frequency command S.sub.SCMD due to the negative feedback of the three-phase primary current.
If it is assumed that the induction motor 13 having an excellent controllability is driven singularly without a load, and the rotor inertia is denoted as J.sub.M, the torque can be expressed as equation (5). ##EQU2##
If the slip frequency S.sub.S is smaller than an ideal value and the torque T.sub.M of the induction motor 13 is insufficient, the speed S.sub.M of the induction motor 13 gradually decreases to cause errors as indicated by equation (5), and the secondary current command I.sub.2CMD is increased by the speed controlling unit 1. The sliding velocity generating unit 3 at the same time increases the slip frequency command S.sub.SCMD, but these operations are not directly related to above mentioned two problems nor are they capable of solving such problems.